Conventional power and free trolley conveyors are characterized by a transport mechanism which includes a trolley mounted on rollers or the like for movement along a track or rail system. Generally, such trolleys have a member extending therefrom which is engaged by a dog member on a chain drive to convey the trolley. In a power and free system, the trolleys may sometimes roll along the tracks freely, even when the dog member on the drive does not engage the trolley. For example, the trolley may be allowed to freely roll along a downhill slope, around a curve, or when being transferred between chain drives. Disengagement between the trolley and the drive dog is desired at points where the trolley is to be stopped, as for example when manufacturing operations, maintenance, or repair access to an object being transferred by the trolley is desired. A carrier member is often mounted on the trolley. The types of carrier members used vary considerably, depending on the nature of the objects to be transported. Various types of carrier members may be used in association with the present invention.
The term "inverted", as in "inverted power and free conveyor", refers to the fact that the drive mechanism which engages the extending member on the trolley runs beneath the track and the trolley. In many conventional systems, the drive mechanism comprises a continuous chain drive with upwardly extending drive dogs thereon. The drive dogs for inverted systems engage a depending member on the trolley to push same. Herein, the depending member on the trolley will be referred to as a trolley dog and the dog on the drive system as a drive dog or pusher dog.
In many systems the trolley dog is vertically movable between upper and lower positions. When in the lower or engaged position, the trolley dog depends low enough to be engaged by pusher dogs being driven underneath the trolley by means of the drive mechanism. On the other hand, when the trolley dog is in the upper or disengaged position, it is generally too high to be reached by the upwardly extending drive or pusher dogs. Therefore, when the trolley dog is in the upper position, the trolley is not positively engaged by the drive dogs and driven.
Many trolley stop arrangements operate by moving the trolley dog between engaged and disengaged positions. For such systems, the trolley dog has a forward cam surface which, when it engages the trolley stop, is cammed upwardly until disengagement from the drive dog is achieved. Typical trolley systems operate by positioning an obstruction in the path of motion of the trolley to urge the trolley dog out of engagement with the pusher dog.
For example, a trolley stop arrangement may utilize a knife blade extending perpendicular to the track of the trolley, sometimes extending completely thereacross, to be engaged by the trolley dog. When withdrawn, the knife blade does not block the path of motion of the trolley dog, and the trolley will, thus, pass the trolley stop under positive drive. On the other hand, when the blade is extended into the path of the trolley dog, it will be engaged thereby, with the camming action lifting the trolley dog and bringing the trolley to a stop.
Such trolley stops, while they have been somewhat effective, have not been completely satisfactory. First, such knife blade arrangements often require portions which extend outwardly from the side of the track or conveyor body. Often a piston for operating the knife blade, a linkage system, or a receptacle for receiving the knife blade extends outward from the side of the track. This not only takes up space which might be more conveniently used but also provides inconvenient and potentially hazardous obstructions to vehicles or personnel moving along the side of the conveyor track. Such systems have not provided for satisfactory, positive braking of the trolley. The trolley comes to a stop primarily because of the friction of the trolley dog rubbing against the knife blade when the trolley dog is disengaged from the drive dog.
U.S. Pat. No. 4,790,247 mentioned above is directed to a trolley stop assembly including a stop arm mounted for pivoting about a transverse axis between a lower trolley non-stopping position and an upper trolley disengaging and braking position. The stop arm is pivoted to the upper position by contact of a pivoted cam with an under side of the stop arm. When the cam is pivoted out from under the stop arm, the stop arm is released to assume its lower position under the influence of gravity. In certain environments, the free pivoting of the stop arm can be degraded by contamination of the stop arm pivot bearing with dirt or other foreign material. For example, in a manufacturing plant in which the trolleys carry parts to be painted, a stop arm pivot bearing of a trolley disengagement and braking assembly near a painting station may become encrusted with paint particles from overspray. When the stop arm is released to allow trolleys to pass by, the stop arm may not fall to its lower position, resulting in unintended disengagement and braking of trolleys.